Falko/Till - install doc correction/addition

Just did a new set up of ispconfig on a lenny system, the docs worked great *except* - in the manual_install file on your website, step 7 not using an encrypted key also applies to step 8 but is not mentioned.

Also, in the perfect server howto series (or maybe just in the ispconfig install doc) a mention of needing libxml2-dev so that the roundcube package provided will work with HTML mail would be great. Would've bit me again, but I remembered from my first install about it...